The Long Term Evolution (LTE) system is in a flattened Radio Access Network (RAN) structure, and requires no Radio Network Controller (RNC). FIG. 1 shows a structure of an LTE system. As shown in FIG. 1, the LIE RAN includes an eNodeB and an Evolved Packet Core (EPC). The eNodeB evolves from the NodeB and the RNC in the R6 stage, and different eNodeB's are interconnected through an X2 interface in the mesh mode. The interface between the eNodeB and the EPC is called an S1 interface. An EPC includes a Mobility Management Entity (MME) and a System Architecture Evolution (SAE) GateWay (SGW). As a control plane part, the MME is responsible for the control plane mobility management, including user context and mobility state management, and allocation of a Temporary Mobile Subscriber Identifier (TMSI). As a user plane part, the SGW is responsible for initiating paging for the downlink data in the idle state, and managing and storing the Internet Protocol (IP) bearer parameters and intra-network routing information, and so on. The MME is connected with the SGW in a mesh mode. That is, one MME controls multiple SGWs. The S1 interface supports many-to-many connection relationships between the EPC and the eNodeB.
FIG. 2 shows a user-plane protocol stack of an LTE system specific to the LTE structure shown in FIG. 1. Generally, all functions of the RNC in the existing network are located to the eNodeB so that the eNodeB has all radio interface protocol stacks. As shown in FIG. 2, the user-plane protocol stack of the evolved architecture of the layer-2 node includes a User Equipment (UE) user-plane protocol stack, an eNodeB user-plane stack protocol, and an SGW user-plane protocol stack. The UE communicates with the eNodeB through a Uu interface, and the eNodeB communicates with the SGW through an S1 interface.
The SGW user-plane protocol stack includes: a GPRS Tunneling Protocol-User plane (GTP-U) layer, a User Datagram Protocol/Internet Protocol (UDP/IP) layer, an L2 layer, and an L1 layer.
The eNodeB user protocol stack includes a radio interface protocol stack and an S1 interface protocol stack. The radio interface protocol stack includes: a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, a Media Access Control (MAC) layer, and an L1 layer. The S1 interface protocol stack includes: a GTP-U layer, a UDP/IP layer, an L2 layer, and an L1 layer.
The UE user-plane protocol stack includes: a PDCP layer, an RLC layer, a MAC layer, and an L1 layer.
The L2 layer refers to layer 2 in the layered protocol, namely, the data link layer, which includes frame relay, Asynchronous Transfer Mode (ATM), or a radio data link layer. The L1 mentioned above refers to layer 1 in the layered protocol, namely, the physical layer, which includes E1, fiber, and microwave transport.
In the S1 handover process in an LTE system, in order to reduce packet loss, the packet forwarding method based on S1 handover is generally applied. However, in the process of forwarding the data, the target eNodeB is unable to ensure the data forwarding of the target eNodeB to be orderly, and is hence unable to ensure lossless migration of packets.